Filters

This article is about filtering, a very powerful facility available to every Linux user, but one which migrants from other operating systems may find new and unusual.

At its most basic level, a filter is a
program that accepts input, transforms it and outputs the
transformed data. The idea of the filter is closely associated with
several ideas that are part of the UNIX operating system: standard
input and output, input/output redirection and pipes.

Standard input and output refer to default locations from
which a program will take input and to which it will write output.
The standard input (STDIN) for a program running interactively at
the command line is the keyboard; the standard output (STDOUT) is
the terminal screen.

With input/output redirection, a program can take input or
send output using a location other than standard input or output—a
file, for example. Redirection of STDIN is accomplished using the
< symbol, redirection of STDOUT by >. For example,

ls > list

redirects the output of the
ls command, which would normally
go to the screen, into a file called
list. Similarly,

cat < list

redirects the input for cat, which
in the absence of a file name would be expected from the keyboard,
to come from the file list--so we
output the contents of that file to the screen.

Pipes are a means of connecting programs together through I/O
redirection. The symbol for pipe is |. For example,

ls | less

is a common way of comfortably viewing the output from a
directory listing where there are more files than will fit on the
screen.

Simple programs provided as standard with your Linux system
can be enhanced by using them as filters for other similar
programs. I'll also show how simple programs of your own can be
built to meet custom filtering needs.

One program I don't look at in this article is Perl. Perl is
a programming language in its own right, and filters are
language-independent.

grep

The program grep, “Get
Regular Expression and Print”, is a good place to begin. (See
“Take Command: grep” by Jan Rooijackers, March 1999.) The
principle of grep is quite simple: search the input for a pattern,
then output the pattern. For example,

grep 'Linus Torvalds' *

searches all files in the current directory for Linus' name.

Various command-line switches may be used to modify grep's
behaviour. For example, if we aren't sure about case, we can
write

grep -y 'linus torvalds' *

The -y switch tells grep to match without
considering case. If you use any upper-case letters in the pattern,
however, they will still match only upper-case. (This is broken in
GNU grep, which simply ignores case when given the
-y switch—that's what the -i
switch does).

With just this bit of information about grep, it is easy to
construct a practical application. For example, you could store
name and address details in a file to create a searchable address
book.

Extended Grep

Sometimes, basic grep won't do. For instance, suppose we want
to find all occurrences of a text string which could possibly be a
reference to Linus. Clearly, searching for 'Linus
Torvalds' is not enough—that won't find just Linus or
Torvalds. We need some way of saying, “This or this or this”.
Here is where egrep (extended
grep) comes in. This handy program modifies standard grep to
provide just such a conditional syntax by using the | character to
denote “or”.

egrep 'Linus Torvalds|L\. Torvalds|Mr\. Torvalds' *

will now find most ways of naming the inventor of Linux. Note
the backslash to “escape” the period. Since it is a special
character in regular expressions, we must tell egrep not to
interpret it as a “magic” character.

tr is perhaps the epitome of
filters. (See “Take Command: A Little Devil Called tr” by Hans de
Vreught, September, 1998.) Short for translate, tr basically does
what its full name suggests: it changes a given character or set of
characters to another character or set of characters. This is done
by mapping input characters to output characters. An example will
make this clear:

tr A-Z a-z

changes upper-case letters to lower-case. A-Z is shorthand
for “all the letters from A to Z”.